1. Field of the Invention
The present invention relates to a process for producing an electroluminescent (hereinafter referred to as “EL”) element comprising an electroluminescent layer formed by photolithography, and an electro luminescent element.
2. Background Art
EL elements cause holes and electrons, injected from opposed electrodes, to be combined within a luminescent layer to emit energy which excites a fluorescent material within the luminescent layer to emit light of a color corresponding to the fluorescent material, and have drawn attention as selfluminous planar display elements.
Among others, organic thin-film EL displays using an organic material as a luminescent material are expected to be applied to advertising or other low-cost simple displays that have high luminescence efficiency, for example, can realize high-brightness luminescence even in an applied voltage of a little less than 10 V, can emit light in a simple element structure, and can realize luminescence display of a specific pattern.
Such organic EL elements generally have a fundamental structure comprising a first electrode layer and a second electrode layer stacked on top of each other through an organic EL layer, and, in general, the first electrode layer and the organic EL layer are patterned to form an element which can realize different luminescent colors.
Patterning processes for organic EL elements are divided into a dry process in which a luminescent material is vacuum deposited through a shadow mask and a wet process in which a luminescent material dissolved in an organic solvent is coated onto predetermined sites by photolithography or ink jet printing. In these processes, patterning by the wet process can eliminate the need to use an expensive vacuum system and thus is advantageous, for example, in that the production cost can be reduced. In particular, the photolithography can realize higher-definition and higher-accuracy patterning as compared with the other methods and thus is expected to be applied to the preparation of high-accuracy organic EL elements.
In general, however, among solvent-soluble organic EL materials used in the wet process, only a very few organic EL materials can be rendered insoluble in solvents by curing/crosslinking treatment. Therefore, the wet process poses a problem that, in patterning organic EL materials different from each other in color on an identical substrate, the already formed organic EL layer is disadvantageously removed by the solvent.
To overcome this problem, Japanese Patent Laid-Open No. 293589/1997 (patent document 1) discloses that an organic EL element comprising different organic EL materials provided on an identical substrate can be prepared by forming an anode (a first electrode layer), an organic EL layer, a cathode (a second electrode layer), and a protective layer in that order on the whole area of a substrate, then providing a photoresist on the protective layer, patterning the photoresist in a desired shape, and then continuously etching the cathode and the organic EL layer at their sites where the photoresist has been removed, by reactive ion etching (RIE).
Further, Japanese Patent Laid-Open No. 69981/1998 (patent document 2) discloses that organic EL layers different from each other in color can be formed on an identical substrate by using materials prepared by dispersing organic EL materials and the like in an ultraviolet curable resin to render an already formed organic EL layer insoluble in the solvent.
Furthermore, Japanese Patent Laid-Open No. 237075/2001 (patent document 3) describes that redissolution of an already formed organic luminescent layer can be prevented by incorporating an organic luminescent material in a specific heat resistant resin and heat curing the mixture.
In the above methods using a binder resin for fixing the organic EL material and the like, however, since the organic EL material is dispersed in the resin, the luminescence efficiency or the service life is disadvantageously lowered. Further, it should be noted that impurities such as a reaction initiator and ions are present as a mixture in the photosensitive resin used as the binder resin. Therefore, in some cases, the properties of the organic EL material and the like are deteriorated by interaction between these impurities and the organic EL material.
For example, Japanese Patent Laid-Open No. 170673/2002 (patent document 4) proposes a method shown in FIG. 4 as a method for forming a plurality of luminescent parts by photolithography.
At the outset, as shown in FIG. 4(a), a luminescent layer 42 is coated onto a substrate 41 provided with an electrode, and, as shown in FIG. 4(b), a photoresist layer 43 is stacked on the luminescent layer. Subsequently, as shown in FIG. 4(c), only a part where a first luminescent part is to be formed is masked by a photomask 44, and the whole photoresist layer except for the part covered by the photomask is exposed to ultraviolet light 45. The assembly is then developed with a photoresist developing solution, followed by washing to remove the photoresist layer 43 in its exposed area as shown in FIG. 4(d). Further, the assembly at its area where the photoresist layer has been removed to expose the luminescent layer is removed by etching to provide an assembly shown in FIG. 4(e).
The above steps can be repeated three times to pattern three different luminescent parts. Finally, peeling treatment is carried out with a photoresist peeling liquid to expose three different luminescent parts 42, 46, and 47 as shown in FIG. 4(n). Thereafter, for example, the step of forming a second electrode layer on each of the luminescent parts is carried out to prepare an EL element which emits light in a direction below the drawing.
In the above method, however, since the first luminescent part in its end part a and the second luminescent part in its end part b are exposed, in coating other coating liquid for a luminescent layer later, disadvantageously, the patterned luminescent part at its end part is eluted in the coating liquid for a luminescent layer which is coated later, and, consequently, cross contamination and a change in film thickness occurs, resulting in luminescence failure.
For example, Japanese Patent Laid-Open No. 45656/2003 (patent document 5) proposes a method for providing a protective layer as shown in FIG. 5 as a method for preventing such cross contamination and preventing pixels from getting narrower.
In this method, as shown in FIG. 5(a), a coating liquid for a first luminescent layer is first spin coated onto the whole area of an assembly comprising a first electrode layer 52 patterned on a base 51 and a buffer layer 53 provided on the first electrode layer 52. The whole area coated coating liquid for a first luminescent layer is dried and cured to form a first luminescent layer 54. A positive-working photoresist is coated on the whole area of the first luminescent layer 54 to form a primary photoresist layer 55 (the step of forming a luminescent layer and a primary photoresist layer). Subsequently, ultraviolet light 57 is applied patternwise using a primary photomask 56 so that a part corresponding to the first luminescent part is not exposed (FIG. 5(a)).
The primary photoresist layer 55 in its exposed part is developed with a photoresist developing liquid followed by washing with water to remove the primary photoresist layer 55 in its exposed part (the step of developing primary photoresist layer). Upon further development with a luminescent layer developing liquid, the luminescent layer 54 only in its part not covered with the primary photoresist layer 55 is removed (FIG. 5(b), step of developing luminescent layer).
Further, as shown in FIG. 5(c), a positive-working photoresist is further coated onto the whole area thereof to form a secondary photoresist layer 55′ (the step of forming a secondary photoresist layer).
A secondary photomask 56′ having a larger width than the primary photomask 56 for masking the primary photoresist layer shown in FIG. 5(a) is used for exposure to ultraviolet light 57 (FIG. 5(d)). The secondary photoresist layer 55 in its exposed part is then developed with a photoresist developing liquid and is washed with water to form a secondary photoresist layer 55′ (protective layer) covering the first luminescent part 54′ and its end part a as shown in FIG. 5(e) (the step of developing secondary photoresist layer).
Thus, in patterning one luminescent layer, carrying out the step of developing photoresist twice, that is, the step of developing a primary photoresist layer and the step of developing a secondary photoresist layer, is advantageous in that, in such a state that the first luminescent part 54′ in its end part a is covered by the photoresist layer, the coating liquid for a second luminescent layer can be coated for next luminescent part formation, that is, second luminescent part formation. Therefore, even when the coating liquid for a second luminescent layer is coated in FIG. 5(f), any problem such as cross contamination does not occur.
The above steps are repeated three times, and the photoresist is then peeled off to form a first luminescent part 54′, a second luminescent part 58′, and a third luminescent part 59′ (FIG. 5(l)). Finally, as shown in FIG. 5(m), a second electrode layer 60 is formed on the luminescent parts to produce an EL element which emits electroluminescent light in a direction below the drawing.
In the above method for forming a secondary photoresist layer (a protective layer) covering the end part a, the cross contamination and narrowing of pixels can be prevented, but on the other hand, the number of steps is large, posing a problem of production efficiency.